The invention relates to a novel tanning agent formulation. The invention also relates to a process for manufacturing semifinished leather products which employs said novel tanning agent formulation.
Chrome tanning has been an important chemical treatment in leather manufacture for over 100 years. For ecological reasons, however, possible alternatives to chrome tanning are being sought. In conventional chrome tanning processes, chromium salts are offered in an amount of 1.5-2.5% by weight of chromium(III) oxide, based on pelt weight, to obtain a leather which resists the boiling test. A considerable portion of the available chromium is neither bound by nor incorporated in the hide tissue and as a result passes into the wastewater.
It is true that chemical treatment of wastewaters (with lime and iron salts) brings about an appreciable reduction in the level of dissolved chromium, but the chromium reappears in the sludge cake which, for disposal, needs to be transported to a special landfill site.
The chrome shavings produced in the course of the leveling of leather represent a similar danger to the environment. The amount produced is relatively large, at 8-15%, based on hide weight, and its disposal is an increasing problem.
Attempts to improve the degree of exhaustion of chrome tanning liquors, and chrome recycling processes, where the residual liquors are reused for tanning directly, or after precipitation and working up to chrome tanning agents, do not overcome the problem of chromium-containing waste materials, for example shavings, trimmings and thin slivers from which no more leather is to be had.
The need of the moment is therefore to propose an alternative process for reducing the chromium content in tannage wastes and wastewater to such an extent that the problems of disposal are largely eliminated.
The possibilities which come into consideration are not very large. Previous attempts to displace chromium salts from their dominating position in tanning by tanning agents of certain environmental acceptability, for example aluminum and zirconium salts, vegetable and synthetic tanning agents or products based on aldehydes, have heretofore not led to any solution which on an industrial scale is completely satisfactory.
In order to retain the generally acknowledged advantages of chrome tanning as regards leather quality while reducing the disadvantages of their waste disposal problems to a substantial extent, the following sequence of operations shall be chosen:
Phase 1: Pretanning without metal salts or formaldehyde
Pretanning is intended to confer adequate thermal stability on the pelts (shrinkage temperature Ts above 70.degree. C.), so that mechanical treatments, such as samming, shaving or splitting, can be carried out satisfactorily. The resulting waste materials shall not pollute the environment.
Phase 2: Variable final tanning
The leather character shall be determined by the appropriate choice of suitable tanning agents for the final tanning of the pretanned, hydroextracted, shaved or split material.
The prior art methods for pretanning rely on the use of known chromium-free products. These include vegetable, synthetic and in particular mineral tanning agents, for example aluminum and zirconium salts. These processes require appreciable amounts of tanning agents in order to achieve adequate stabilization of the hide tissue. The disadvantages here include appreciable pollution of the wastewater and the production of wastes which are difficult to utilize or dispose of, and adverse effects on the leather character.
There is some hope in the use of aldehydes in pretanning. The tanning effect of aldehydes and dialdehydes has been known for a long time. It has also been previously pointed out that the amounts required for adequate final tanning are relatively small. (See Herfeld, H., "Bibliothek des Leders", volume III, page 191, Umschau Verlag, Frankfurt/Main 1984).
In practice, the good tanning properties of glutardialdehyde make it the predominant aldehyde these days. In pretanning, however, the use of glutardialdehyde is difficult. If relatively small amounts of glutardialdehyde are used (0.5-0.8%, based on 100% aldehyde and based on the pelt weight), it is in general impossible to obtain shrinkage temperatures above 70.degree. C. The resulting semifinished products are difficult to hydroextract. During shaving, the flesh side frequently undergoes denaturation (gelatinization), which has an adverse effect on the quality of the finished leather.
Measurements of the shrinkage temperature across the entire cross-section of the semifinished product show an interesting and revealing distribution (see FIG. 1).
The relatively low shrinkage temperatures in the middle layer of the semifinished product explain the difficulties observed in practice. The distribution of the shrinkage temperature becomes more and more uniform with increasing supplies of glutardialdehyde. At 1.5-2.0%, based on 100% strength aldehyde based on pelt weight, only small differences appear. The semifinished products thus obtained, however, constitute largely fully tanned leather.
The irreversible crosslinking of the hide tissue predetermines the final character of the finished leather and no longer permits any subsequent variable processing as desired by the tanner.
An improvement in the effectiveness of glutardialdehyde at lower concentrations (0.3-0.8%, based on 100% strength aldehyde and based on pelt weight) can only be expected from a more uniform distribution of the tanning molecules in the hide cross-section. To obtain diffusion, the astringent components must be inactivated reversibly, so that in the first pretanning phase crosslinking is suppressed and penetration is facilitated. Since the crosslinking of the collagen fibers with dialdehydes, in particular glutardialdehyde, is due not to a single compound, but to a plurality of compounds of which a part are still unknown (see Heidemann, E., et al., Leder 25 (12), 229 (1974); Anderson, P. J., J. Histochem. Cytochem. 15, 652 (1967); Robertson, A. A., et al., J. Ultrastruct. Res. 30, 275 (1970); Meek, M. K. et al., J. Mol. Biol. 185, 359 (1985); Tashima, T. et al., Chem. Pharm. Bull. 35, 4169 (1987)), the reversible stabilization should have a universal function.